1. FIELD OF THE INVENTION
This invention is directed to a unique double-loop SO.sub.2 scrubbing system for flue gas desulfurization which achieves substantially complete lime or limestone utilization concurrently with high SO.sub.2 scrubbing efficiency, and excellent corrosion control.
2. DESCRIPTION OF THE PRIOR ART
Scrubbing of boiler flue gases with slurries of limestone (CaCO.sub.3) or calcined limestone products, lime and hydrated lime, is a known and simple method for the removal of sulfur dioxide (SO.sub.2) from these combustion gases. The system does, however, require makeup water for operation and therefore increases the total plant demand for water. As suitable quality water is often available to the plant only in limited quantities, it is essential that the scrubbing system use a minimum of high quality makeup water.
Makeup water is required in sulfur dioxide scrubbing systems to replace water lost in two areas: (1) water lost through evaporation as a means of saturating the incoming gases; and (2) water lost with the solid waste product composed of unreacted reagent, calcium sulfite hydrates and calcium sulfate hydrites that are discharged from the system. The total makeup water requirements for the system can, therefore, be minimized by reducing these water losses.
Generally makeup water enters the sulfur dioxide scrubbing system in several areas: (1) water entering with the reagent; (2) water for slurry pump packing glands; (3) wash water to remove solids that have built up on impingement-type demisters; (4) quencher makeup water, which replaces most of the evaporative losses. Most of the makeup water is either demister wash water or quencher makeup water. The water used as demister wash water must be of sufficiently high quality so as not to contribute to the solids build-up it is designated to wash away. Its solids content and pH must be kept within limits. The quencher makeup water, on the other hand, can be of considerably lesser quality.
One prior art method for minimizing system water losses (i.e. process makeup water requirements) is to operate the scrubbing system in a closed loop manner. In closed loop operation, the discharge slurry from the scrubbing system is partially dewatered to generate two new streams: a high solids stream that is sent to disposal; and a low quality water stream that is recycled to the scrubbing system.
Utilization of this recycled water can reduce the overall process fresh makeup water requirements by up to 50%. However, the recycled water is saturated in sulfates - which increases the potential for scaling in the flue gas desulfurization system. In addition, impurities including corrosive chlorides are concentrated within the SO.sub.2 Removal System during closed loop operation. These impurities force the use of exotic and expensive materials of construction and often preclude closed loop operation in single loop desulfurization systems.